The invention relates generally to formation fluid sampling. More particularly, the invention relates to a method and an apparatus for obtaining a fluid sample from a subsurface formation traversed by a borehole while controlling the flow rate and/or pressure.
Fluid samples from subsurface formations are typically collected from a reservoir for analysis at the surface, downhole or in specialized laboratories. Information obtained from analyzing formation fluid samples often plays a vital role in the planning and development of hydrocarbon reservoirs and in the assessment of a reservoir""s capacity and performance.
FIG. 1 shows one example of a conventional formation testing tool 100 which may be used to obtain a sample or conduct tests in a subsurface formation. Sampling operations are typically conducted in xe2x80x9coverbalancedxe2x80x9d boreholes, wherein the hydrostatic pressure of the borehole fluid is greater than the formation pressure. Overbalancing typically prevents the formation fluid from breaking through the walls of the wellbore and causing either xe2x80x9cblowoutsxe2x80x9d or undesired pressure at surface.
In a typical sampling operation, the formation testing tool 100 is lowered into an overbalanced borehole 109 on a wireline 111 and positioned adjacent the subsurface formation 103 to be sampled. The formation testing tool 100 makes physical contact with the inside surface of the borehole 109 by engaging a probe 104 of a probe assembly 102 with a wall 112 of the borehole 109. One or more stabilizer pads 115 also extend from the formation testing tool 100 to stabilize the formation testing tool 100 in the borehole 109.
As shown in FIG. 1, the formation testing tool 100 includes a pump module 105 which is used to induce fluid flow from the formation 103 into the formation testing tool 100. An analyzer module 106 may also be provided to analyze fluid obtained from the formation. A plurality of sample tanks (not shown) are also disposed in a sample tank module 118 of the formation testing tool 100 to enable the collection of formation fluid samples in the tool 100.
Contact between the probe 104 of the formation testing tool 100 and the borehole wall 112 enables pressure communication with the formation 103. A seal is disposed around the probe 104 to isolate the inner parts of the formation testing tool 100 from the borehole fluid. In openhole boreholes, mudcake is typically disposed on the borehole wall 112 to isolate the formation fluid from the borehole fluid. In cased boreholes, casing and cement are disposed in the borehole to isolate the formation fluid from the borehole fluid.
Once the formation testing tool 100 is positioned and set as described above, one or more formation fluid samples may be obtained from the formation 103. Fluid communication is established between the formation testing tool 100 and the subsurface formation 103 by contacting the probe 104 to the subsurface formation 103. Because the formation 103 is at a lower pressure than the borehole 109, and the formation testing tool 100 is in communication with the higher borehole pressure, formation fluid may then be drawn into the formation testing tool 100 by using a downhole pump module 105. A downhole pump is used to create a desired pressure differential between the formation testing tool 100 and the subsurface formation 103 to induce flow from the formation 103 into the formation testing tool 100.
Other prior art formation testing tools and sampling methods have been developed as described in detail in U.S. Pat. Nos. 4,860,581; 4,936,139 (both assigned to Schlumberger); U.S. Pat. No. 5,303,775 (assigned to Western Atlas); and U.S. Pat. No. 5,934,374 (assigned to Halliburton). The formation sampling methods and tools in these cases disclose formation sampling operations carried out by flowing fluid into the formation testing tool with a downhole pump that creates a desired pressure differential. U.S. Pat. No. 5,377,755, assigned to Western Atlas International is another example of a formation testing tool used for sampling. This patent describes a formation testing tool including a bi-directional pump adapted to control the pressure differential in sample tanks. Valves are disposed in flow lines between the pump and the sample tanks to allow for the selective communication of fluid therebetween.
The prior art downhole testers and sampling techniques utilize pumps to collect samples and maintain the samples in xe2x80x9csingle phase.xe2x80x9d In single phase sampling operations, the pressure drop experienced by the formation fluid must be minimized to avoid drawing the formation fluid sample at a pressure below its bubble point pressure or asphaltene precipitation point. This is achieved in prior art formation testing tools by providing flow control during sampling. The flow control is largely dependent on the operation of one or more downhole pumps. As formation fluid is drawn out of the formation, the pressure drop experienced by the formation fluid and the rate of flow are regulated by the speed of the pump.
In a sampling operation, the initial drawdown of formation fluid from the formation is often contaminated by mudcake, filtrate, or debris. Pumps are used to remove a sufficient amount of formation fluid before collecting a formation fluid sample to purge these contaminates from the fluid stream. This initial formation fluid removal operation is referred to as the clean-up phase. When a sampling operation includes a clean-up phase, flow control is provided downhole by initially running a downhole pump as fast as possible to reduce the clean-up period and then lowering the downhole pump speed to maintain the formation fluid sample in a single phase during collection or downhole analysis of the sample. If the speed required by the downhole pump is below a certain operating threshold, the pump motor may stall causing the pump to fail. Therefore, the operating range of the downhole pump must be optimally designed or selected prior to a sampling operation. If failure of the downhole pump occurs during an operation, either another pump is required or the tool must be pulled to the surface and the existing pump fixed or replaced before a single phase sample may be acquired.
To minimize or avoid problems associated with the use of downhole pumps during sampling operations, a method is desired which allows for a formation fluid sample to be obtained and that allows for control of the flow rate and/or pressure disturbance experienced by the formation fluid during sampling. A method is also desired which permits sampling in a wellbore which does not require the use of a downhole pump. It is further desired that such a method may provide a technique for obtaining single phase samples.
In one aspect, the present invention relates to a method for sampling a subsurface formation traversed by a borehole. In one embodiment, the method comprises positioning a formation testing tool in a borehole having borehole fluid therein with a pressure less than formation pressure such that a pressure differential exists between the formation and the borehole. The formation testing tool includes a sample chamber having a first side, a second side and a movable fluid separator disposed there between. The method further includes establishing fluid communication between the formation testing tool and the formation and inducing fluid flow from the formation to the formation testing tool by exposing an interior of the formation testing tool to the pressure differential. The method also includes capturing a sample of the formation fluid in a sample tank associated with the formation testing tool by exposing the sample tank to the pressure differential.
In another aspect, the present invention relates to a method for performing a controlled pretest on a subsurface formation traversed by a borehole. In one embodiment, the method comprises positioning a formation testing tool in a borehole having borehole fluid therein with a pressure less than formation pressure such that a pressure differential exists between the borehole and the formation. The formation testing tool includes a variable volume sample tank having a sample chamber, a buffer chamber, and a moveable fluid separator between the sample chamber and the buffer chamber. The method further comprises establishing fluid communication between the formation testing tool and the formation, and inducing fluid flow from the formation into the formation testing tool by exposing an interior of the formation testing tool to the pressure differential. The method also includes drawing a volume of formation fluid in the sample tank by directing the formation fluid to the sample chamber of the sample tank and exposing the buffer chamber of the sample tank to the borehole pressure. The method further includes holding the volume on the sample chamber of the sample tank constant to allow pressure in the sample tank to build-up to a pressure proximal the formation pressure.
In another aspect, the present invention relates to a system for pressure controlled downhole sampling a subsurface formation traversed by a borehole. In one embodiment, the system comprises a formation testing tool adapted for placement in the borehole and a wellhead. The wellhead is disposed about the borehole proximal the surface and is adapted to seal borehole fluid therein such that the borehole fluid is maintained at a desired pressure. The formation testing tool includes a probe assembly, a conduit system, and at least one sample tank. The probe assembly is adapted to establish fluid communication between the formation testing tool and the subsurface formation. At least one sample tank includes a sample chamber adapted to accept formation fluid therein, a buffer chamber in fluid communication with the borehole, and a moveable fluid separator disposed between the sample chamber and the buffer chamber to maintain a separation of fluid there between. The conduit system includes a first end in fluid communication with the probe assembly, a second end in fluid communication with the borehole, and a third end in fluid communication with the sample chamber of the sample tank. The wellhead includes a sealing apparatus disposed about the borehole and adapted to seal borehole fluid therein, at least one pressure increasing device disposed in fluid communication with the borehole and adapted to enable selective increase of pressure in the borehole, and at least one flow adjustment device adapted to enable adjustment of the flow of borehole fluid out of the borehole.
Advantages of one or more embodiments of the invention may include the ability to accurately control the pressure drop experienced by the formation fluid during sampling by manipulating surface pressure applied to the borehole at the surface. Advantageously, by controlling the pressure and/or flow rate of borehole fluid at the surface a single phase formation fluid sample may be obtained.